The present invention relates to certain compositions and processes useful for minimizing fouling by manganese deposition in aqueous systems. More particularly, the present invention relates to deposit control agents and chemical water treatments adapted to stabilize manganese against oxidation.
Fouling by manganese deposition in pipelines and waterworks has been a concern for some time and it continues to impact water supplies today. Manganese deposition in cooling water circuits degrades corrosion resistance, lowers heat exchanger efficiency, and reduces biocide performance. The detrimental effects of manganese deposition inflict significant costs to the electric power industry through increased fuel consumption, more frequent and extensive clean-ups, higher chemical treatment costs, and in some cases, significant capital costs for component replacement. In addition to cooling water circuits, manganese deposition may occur in a variety of other aqueous systems including drinking water distribution systems, sewage treatment plants, swimming pools, laundries, bottling plants, air scrubbers, and car washes. Problems range from corrosion and blockage of pipes and nozzles, to staining, discoloration, and inferior taste. The adherent and oftentimes coarse deposits that form from manganese deposition can adversely affect heat transfer, induce abrasive wear, and reduce pumping efficiency. Removal of manganese deposits often requires aggressive remedial techniques, such acid leaching and mechanical cleaning.
In water systems, manganese promotes pitting and crevice corrosion through a combination of electrochemical effects caused by galvanic coupling between manganese deposits and the underlying metallic surface. The effect of the galvanic action is to shift the metal corrosion potential in the positive or noble direction fostering passive film breakdown and localized corrosion. The effect is most severe for stainless steels, but also occurs on brass and other copper alloys. Regions in the U.S. where stainless steel or copper alloy corrosion has been linked to manganese deposition include Northern Virginia, the Ohio River Valley, central Maine, eastern Nebraska, S.C., and the Gulf Coast region.
Concerns over manganese fouling are typically less than concerns related to calcium, silica, or iron due to the often low or undetectable levels of manganese in the majority of commercial and residential water supplies. Many surface and groundwaters nevertheless have manganese levels that pose a significant fouling threat. Corrosion and other adverse effects of manganese deposition in systems serviced by such water can lead to equipment repair and replacement expenses that far exceed costs associated with the more prevalent mineral scalants.
Deposition of manganese in oxic, neutral-to-alkaline waters is caused by the oxidation of soluble divalent manganese into water-insoluble manganese oxides and oxyhydroxides. The insoluble, higher valent manganese compounds are loosely grouped and referred to simply as manganese dioxide or MnO2. MnO2 can form when manganese-containing water is exposed to halogens, ozone, or other chemical oxidants that are commonly used for disinfection purposes in water systems. Alternatively, MnO2 can be produced by a wide range of microorganisms that grow naturally in surface and in groundwaters worldwide. Microbial reactions scavenge manganese in a highly efficient manner, enabling waters containing as little as 20 ppb dissolved manganese to deposit visible MnO2 deposits within as little as a few days' time.
Methods to control manganese deposition can be categorized as either 1) manganese removal or 2) manganese stabilization. Dissolved manganese is typically removed from water supplies by chemical oxidation followed by filtration. The oxidation/filtration approach uses permanganate, chlorine dioxide, chlorine, ozone or other oxidants to convert soluble manganese into insoluble MnO2, followed by settling or filtration of the solid material. Chemical oxidation is highly effective in removing both iron and manganese, as well as lowering organic carbon levels and reducing halogen demand in the water system. However, high capital and operating costs coupled with the risk of aggressive oxidant carry-over make these approaches impractical for many applications. Spray ponds and aeration chambers commonly used to remove iron are not effective in removing manganese due to the much slower kinetics of manganese air-oxidation.
Manganese stabilization, on the other hand, is achieved by using chemicals to interfere with the formation and growth of MnO2 particles. The use of polyphosphate in drinking water systems for this purpose is well established. The formation of a manganese polyphosphate complex inhibits deposition. Threshold inhibitors such as phosphonic acid derivatives are used to maintain particulate MnO2 in a colloidal, more easily dispersed state, by coupling to and preventing growth at active sites on the particle surface. In combination, polymeric dispersants (including polyacrylates and multifunctional copolymers), are used to disperse the colloidal MnO2 and prevent its aggregation and settling.
Halogenation and alkaline pH degrade the effectiveness of the threshold inhibitor/polymeric stabilization technique by increasing the rate of manganese oxidation, and, in the case of halogen, by directly breaking down the inhibitor and dispersant molecules. The effectiveness of the treatments is also diminished by the presence of high levels of hardness, silt, iron, or other suspended solids that compete for both the threshold inhibitor and the dispersant. Further, the level of manganese deposition control provided by phosphonate/polymeric treatments alone has an upper limit, beyond which, increased doses or concentrations of phosphonate/polymer do not result in increased manganese control. The upper limit of effectiveness is particularly problematic in water systems having manganese in a relatively high total amount and/or operating at high cycles of concentration.
The reaction of Mn(II) with chlorine can be accelerated by adsorption of Mn(II) onto the growing MnO2 surface, which acts as a catalyst for further oxidation. This “autocatalytic” effect improves the efficiency of manganese removal systems; however, it adversely affects efforts to stabilize dissolved manganese against oxidation by halogens. MnO2 that forms on surfaces by microbiological oxidation of trace levels of dissolved manganese serves as active material that “seeds” further chemical MnO2 deposition. The seeding can lead to MnO2 deposition under conditions that might not otherwise support chemical manganese oxidation. Accordingly, it is an objective of chemical water treatment to stabilize manganese against oxidation by halogens so that the oxidant can be used to inhibit microbial growth without risking chemical deposition of manganese.
A correlation may exist between the behavior of manganese and the behavior of cerium. For example, the oxidation of manganese in marine waters may be inhibited by the presence of cerium. The observations relate to the formation of manganese dioxide by natural oxidation processes. Cerium has also become the focus of significant corrosion research as a possible, preferred corrosion inhibitor to replace toxic chromium compounds. Neither relate to the inhibition of manganese deposition caused by chemical oxidation. Phosphonic acid derivatives with metal ions, including ceric ions (i.e., cerium in the +4 oxidation state), can be used for corrosion inhibition and scale control in aqueous systems, without specifying the control of manganese, and further, without teaching the use of cerium for protection against the oxidation of manganese by oxidizing chemicals.
Accordingly, a need exists to control chemical oxidation of manganese by halogen compounds that are commonly applied for microbial control and sanitation in aqueous systems. A need also exists to enhance the level of manganese deposition control heretofore provided by widely used deposit control agents. A further need exists to control the degradation of organic additives, such as polymer and phosphonate additives, from exposure to chlorine and other halogen compounds.